add options for custom lunr Liquid and JS code (#1068)

This is a prototype for review and discussion. My use and testing of this PR is on top of 6d9d41359c. The changes are trival to rebase to `main` and I'm happy to do so if this prototype moves forward.


* Feature request details in linked issue, fixes just-the-docs/just-the-docs#1067
* I welcome feedback and all discussion
* A draft doc site of mine using this PR is at https://docs.hidale.com/

To use the prototype, the two include files need to be customized. Here are mine from the draft website 9c0d836408

Co-authored-by: Matt Wang <matt@matthewwang.me>

assets/js/just-the-docs.js

@@ -87,6 +87,7 @@ function initSearch() {
         this.metadataWhitelist = ['position']
 
         for (var i in docs) {
+          {% include lunr/custom-index.js %}
           this.add({
             id: i,
             title: docs[i].title,

assets/js/zzzz-search-data.json

@@ -51,6 +51,7 @@ permalink: /assets/js/search-data.json
     "title": {{ title | jsonify }},
     "content": {{ content | replace: '</h', ' . </h' | replace: '<hr', ' . <hr' | replace: '</p', ' . </p' | replace: '<ul', ' . <ul' | replace: '</ul', ' . </ul' | replace: '<ol', ' . <ol' | replace: '</ol', ' . </ol' | replace: '</tr', ' . </tr' | replace: '<li', ' | <li' | replace: '</li', ' | </li' | replace: '</td', ' | </td' | replace: '<td', ' | <td' | replace: '</th', ' | </th' | replace: '<th', ' | <th' | strip_html | remove: 'Table of contents' | normalize_whitespace | replace: '. . .', '.' | replace: '. .', '.' | replace: '| |', '|' | append: ' ' | jsonify }},
     "url": "{{ url | relative_url }}",
+    {% include lunr/custom-data.json page=page %}
     "relUrl": "{{ url }}"
   }
         {%- assign i = i | plus: 1 -%}
@@ -62,6 +63,7 @@ permalink: /assets/js/search-data.json
     "title": {{ page.title | jsonify }},
     "content": {{ parts[0] | replace: '</h', ' . </h' | replace: '<hr', ' . <hr' | replace: '</p', ' . </p' | replace: '<ul', ' . <ul' | replace: '</ul', ' . </ul' | replace: '<ol', ' . <ol' | replace: '</ol', ' . </ol' | replace: '</tr', ' . </tr' | replace: '<li', ' | <li' | replace: '</li', ' | </li' | replace: '</td', ' | </td' | replace: '<td', ' | <td' | replace: '</th', ' | </th' | replace: '<th', ' | <th' | strip_html | remove: 'Table of contents' | normalize_whitespace | replace: '. . .', '.' | replace: '. .', '.' | replace: '| |', '|' | append: ' ' | jsonify }},
     "url": "{{ page.url | relative_url }}",
+    {% include lunr/custom-data.json page=page %}
     "relUrl": "{{ page.url }}"
   }
         {%- assign i = i | plus: 1 -%}